Sol-gel Template Synthesis and Liquid CO 2 Developed TiO 2 /CdS Composite Nanowire Arrays
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Sol-gel Template Synthesis and Liquid CO2 Developed TiO2/CdS Composite Nanowire Arrays Bin Cheng, Clarence W. Murray, and Edward T. Samulski* Department of Chemistry, CB#3290 University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3290, U.S.A. E-mail: [email protected] ABSTRACT TiO2/CdS composite nanowire (~ 200 nm diameter and ~ 50 µm length) arrays were successfully prepared by the sol-gel template method. An anticipated good nanowire arrays was obtained by using liquid or supercritical CO2 to develop the target materials in purification process. The morphology, crystallinity, and composition of the product was characterized by SEM, XRD, and EDX. INTRODUCTION Since the successful growth of carbon nanotubes, one-dimensional materials have received increasing attention, both because of their fundamental importance and their wide-ranging potential applications in nano devices [1-4]. Fabrication of nanoscale heterogeneous composites offers additional possibilities in developing novel functional materials. The study of interparticle electron transfer between dissimilar semiconductor particles has attracted much interest [5,6]. Combining two semiconductor particles together offers an opportunity to sensitize a semiconductor material having a large band gap and energetically low-lying conduction band by using a second semiconductor having a small band gap and an energetically high-lying conduction band [7,8]. Oxide semiconductors with a sufficiently negative conduction band, such as TiO2 (Eg = 3.2eV), generally have too large band gap energies to utilize the main mass of the solar energy spectrum [9]. Semiconductors with a somewhat smaller band gap, such as CdS (Eg =2.6eV), have been investigated embedded in TiO2 to study photosensitization. Charge transfer from one semiconductor into another can lead to efficient and longer charge separation, which is anticipated to have potential applications in photocatalysis and solar energy conversion [10]. Several different strategies for fabrication of semiconductor nanoparticle composite films have been reported, including Langmuir-Blodgett (LB) techniques, self-assembly, and mechanical mixing method [10-12]. To our knowledge, there are no reports of large- and small-band gap composite semiconductor nanowires. Although there are some reports that use sol gel method for preparation of individual component semiconductors, the results are not ideal. Lei et al [13] reported fabrication of TiO2 nanowire arrays, but they could not get very good arrays: all of the nanowires are bundled. Cao et al [14] used the sol gel technique to prepare CdS nanowires arrays, but from their experimental procedures, it is impossible to get a stable sol, and furthermore, many impurities would be introduced into the final product. This is the first report using sol-gel template method and liquid CO2 development to successfully fabricate high purity TiO2/CdS composite nanowires in anticipated arrays.
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EXPERIMENTAL DETAILS Titanium (IV) isopropoxide, cadmium acetate hydrate were purcha
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